Reference
Battipaglia, G., Saurer, M., Cherubini, P., Calfapietra, C., McCarthy, H.R., Norby, R.J. and Cotrufo, M.F. 2013. Elevated CO2 increases tree-level intrinsic water use efficiency: insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites. New Phytologist 197: 544-554.
Background
The authors write that "elevated CO2 increases intrinsic water use efficiency (WUEi) of forests," but they say that "the magnitude of this effect and its interaction with climate is still poorly understood."
What was done
In order to better illuminate the nature and magnitude of this phenomenon, Battipaglia et al. combined tree ring analyses with carbon and oxygen isotope measurements made at three Free Air CO2 Enrichment (FACE) sites - POP-EUROFACE in Italy, Duke FACE in North Carolina (USA), and ORNL in Tennessee (USA) - in order "to cover the entire life of the trees," which feat they accomplished using "δ13C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO2 effects on stomatal conductance were explored using δ18O as a proxy."
What was learned
In the words of the seven scientists who conducted the work, "across all the sites, elevated CO2 increased 13C-derived water-use efficiency on average by 73% for Liquidambar styraciflua [POP-EUROFACE, +200 ppm CO2], 77% for Pinus taeda [Duke FACE, +200 ppm CO2] and 75% for Populus sp. [ORNL, +153 ppm CO2], but through different ecophysiological mechanisms."
What it means
Battipaglia et al. say their findings provide "a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modeling elevated CO2 and climate impacts on forest productivity, carbon and water balances." And their results clearly indicate that the CO2-induced increases in forest WUEi they documented are huge.